Plenty of folks were shocked and surprised to see exes Rihanna and Chris Brown kiss on the lips and embrace inside the Staples Center at Thursday's MTV Video Music Awards in Los Angeles -- none more so than Brown's current girlfriend, Karrueche Tran.

A source tells Us that the aspiring model, 23, is "totally pissed off" with her beau of nearly a year after he congratulated Rihanna for her Video of the Year win with a very public show of affection.

"She felt the kiss was a public slap in the face," the source says, adding that Brown decided to attend the VMAs last minute, without letting Tran know. Earlier this year, multiple sources told Us Weekly that Brown, 23, and Rihanna, 24, have been secretly hooking up; the twosome split in February 2009 following his brutal assault of the "We Found Love" singer.

Rihanna recently opened up about her complicated feelings for Brown to Oprah Winfrey, calling him "the love of my life."